The present invention relates generally to the drilling of a hole in a support structure and, more particularly, to a tool and method for forming a hole having an increased cross-sectional area in a region remote from an exposed surface of the support structure so as to form undercut surfaces which facilitate the anchoring of a fastening element therein.
Various fastening elements are already known for the purpose of anchoring an object to a support structure. One type of fastening elements has spreading portions which include an expansion sleeve of generally cylindrical configuration prior to the insertion and anchoring of the fastening element in a cylindrical hole of a support structure, and a screw accommodated in the sleeve. When the screw is tightened, the spreading portions of the sleeve spread apart and anchor the sleeve, thus anchoring the fastening element in the supporting structure.
The spreading of the spreading portions may be achieved either by the screw itself, or by an expander element mounted on the screw which is drawn between the spreading portions and which pushes them apart during the tightening of the screw.
Such arrangements generally achieve satisfactory results provided that the material of the support structure is such as to permit at least partial yielding of the walls surrounding the end of the cylindrical hole in the region in which the spreading portions of the sleeve are accommodated. In this event, the spreading portions of the sleeve deform the walls in this region, thus forming a V-shaped or a conically-enlarged portion in the cylindrical hole so that the spreading portions engage behind the material of the support structure. It will be understood that the anchoring action of the fastening element is proportional to the degree to which the spreading portions of the sleeve are spread apart for any given material of a support structure. Aside from the force available for tightening the screw, the ridigity of the material used for the spreading portions is the most determinative factor which determines the degree of spreading of the spreading portions.
However, experience has shown that when the conventional fastening elements are anchored in cylindrical holes which are provided in masonry walls or concrete structures or the like, the anchoring effect is rather unsatisfactory due to the fact that even when steel is used for the fastening element, the spreading of the spreading portions of the sleeve is limited by the inner circumferential wall of the cylindrical hole itself since the material of the support structure will yield only slightly with disproportionately increasing force being needed for tightening the screw of the fastening element and spreading apart the spreading portions thereof. Furthermore, even if an unlimited tightening force were available, the spreading portions of the sleeves themselves become deformed rather than the highly rigid walls surrounding the hole.
Hence, it has been proposed to form the cylindrical holes with an increased cross-sectional area in its bottom region prior to insertion of the fastening element into the hole. This increased area provides undercut abutment surfaces which diverge in direction away from the exposed surface of the support structure and which are engaged by the spreading portions of the fastening element. However, the presently known tools utilize rigid drill shafts upon which are mounted spherical collars. The collar may have a larger diameter than the hole being drilled in which case the collar rests against the access edge of the hole; alternatively, the collar may have a smaller diameter than the hole being drilled in which case the collar is accommodated in the interior of the hole. In this event, the rigid drill shaft is tilted because of the spherical outer contour of the collar to produce the undercut surfaces.
However, this prior-art technique has not proven altogether satisfactory since the rigid drill shaft has a tendency to break as it is rocked in the hole. The lever-type action causes the drill shaft to be especially subject to breakage when it is desired to produce undercut surfaces which extend greatly into the interior of the support surface in the radial direction.
There are also already known various fastening elements which are anchored in a support structure by injecting a hardenable substance, such as cement, into the interior of the cylindrical hole exteriorly of the fastening element so as to form plug thereabout. The anchoring values for such known fastening elements primarily depend upon the composition of the hardenable substance itself.